Choose the correct option. My mum _______ on computer while my dad _______ on the phone.

The correct option is 'was working / was talking'. The sentence requires the use of the past continuous tense to describe actions happening at the same time in the past. The phrase 'while' signals that two actions were ongoing simultaneously, hence both parts of the sentence need to be in the continuous form. Thus, 'was working' for the mother's action and 'was talking' for the father's action.